The present invention relates to a system for controlling a four-cycle automobile internal combustion engine and more particularly to a valve duration control system for such an engine.
In controlling a four-cycle automobile internal combustion engine, it is known from JP-A 62-191636 to retard timing at which an intake valve opens and timing at which the intake valve closes to decrease an overlapping between exhaust and intake periods when engine speed is lower than a predetermined engine speed value and engine load is less than a predetermined engine load value to improve combustion efficiency to fit engine operation at low speed with low load, and to advance timing at which the intake valve closes toward bottom dead center (BDC) as well as timing at which the intake valve opens when engine speed is lower than the predetermined engine speed but the engine load is not less than the predetermined engine load to enhance charging efficiency for increased output torque during engine operation at low speed with high load. JP-A 62-191636 proposes use of a variable valve timing mechanism, employing a single cam profile, to shift the timings of the intake valve with valve duration unaltered.
JP-A 2-42105 proposes to conduct a shift between valve actuation by low speed cam and valve actuation by high speed cam when engine output produced by valve timing of low speed cam is generally equal to engine output produced by valve timing of high speed cam in order to suppress shocks during the shift.
According to JP-A 62-191636, the valve closing timing remains delayed with respect to BDC during a shift from engine operation at low speed with low load to engine operation at low speed with middle load that is less than the predetermined engine load value, causing unsatisfactory charging efficiency and poor acceleration. In case of rapid acceleration to cause an increase from low engine load to high engine load that exceeds the predetermined engine load value during operation at low speed, a sufficiently quick increase in charging efficiency is not expected owing to a delay in hydraulic actuator of the variable valve timing mechanism, causing unsatisfactory performance at rapid acceleration.
According to the control strategy taught by JP-A 2-42105, a difference in engine torque during a shift can be suppressed. However, the shift progresses during a valve closing period within one revolution of camshaft. In other words, the torque difference occurs during a very short period of time. Thus, it is difficult to suppress torque shock to a satisfactorily low level. Besides, according to this known control strategy, the shift is prohibited unless engine output produced by valve timing of low speed cam is generally equal to engine output produced by valve timing of high speed cam. Specifically, the shift is prohibited during engine operation at low speed.
An object of the present invention is to improve a valve duration control system of a four-cycle automobile internal combustion engine such that acceleration performance from engine operation at low speed with low load to engine operation at low speed with high load is improved and production of NOx is suppressed during engine operation at low speed with middle load.